Hy i just saw this Question: How does a rolling code work?

And I just wondered how it is possible that my Vehicle (A 2013 Mazda 6) has two key fobs? As i understand this shuld not be possible. Because there shuldbe only one key that stores n+1.

Are these Vehicle using another form of a Rolling Key Algorithm or are there Keyfobs that do full handshake?(Rx+Tx)

up vote 0 down vote accepted

Keyfobs that do a handshake are actually quite common, see the HiTag2 key transponder specifics for how its handshake works.

Even without a handshake, the code can still identify the key if it is long enough, and cars can be programmed to accept multiple keys, but almost certainly there is a handshake involved.

I believe your key probably uses a Ford type transponder chip, probably transmitting at 315MHz for long range commands like door unlock, and a lower freq for engine start (123-134KHz). For engine start, the car tells the key it wants to start and adds a challenge code, then sends to the key. The key encrypts the challenge, resulting in an 80-bit code, then truncates to 40-bits and transits back with its serial number and a checksum. The car then verifies the code, and starts if it is correct.

I am not sure if the high frequency commands use the same cipher, but they are a 1-way transmission. Similar to how a garage door opener can use 2 remotes, the car can use 2 keys to unlock doors without a handshake, as internally the car will see that the code matches a programmed key.

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